(1) Field of the Invention
This invention relates to optical telecommunication technology, particularly to an optical spectral encoder/decoder for a Code Division Multiple Access (CDMA) communication system.
(2) Brief Description of the Related Art
Fiber-optics communication links are gaining great popularity because the transmission medium has a comparatively unlimited bandwidth and excellent attenuation properties.
In a broadcast and select communication system, it is desirable that a number of end-user stations be able to interconnect their respective communication links through a common bus. This ability is known as multiple access. Two common multiple access systems are the time division multiple access (TDMA) and the Code Division Multiple Access (CDMA). In a TDMA system, each user is assigned a time slot. In a CDMA system, each user is assigned a special code. Signals are separated using a correlator that accepts only signal energy from the key variable binary (sequence) code used at the transmitter.
An optical CDMA (O-CDMA) system is amenable to optical communication, because the codes can be represented by different wavelengths (colors) of light. Signals can be spectrum-encoded.
Recently there has been considerable interest in CDMA systems as an efficient protocol for local area broadcast networks. Both coherent and incoherent systems have been investigated. Coherent approaches require pico- or femtosecond pulsed lasers and phase detection systems that are generally complex, expensive, and cumbersome. Several different incoherent schemes have been proposed and demonstrated, some of which implemented bipolar coding as used in radio frequency CDMA and spread spectrum systems. The most successful incoherent systems employ complementary spectral keying (CSK), in which one spectral pattern is transmitted for data xe2x80x9c1xe2x80x9d and the complementary pattern is transmitted for data xe2x80x9c0xe2x80x9d. CSK is a very powerful and general modulation technique that, independent of the particular coding structure used, can provide significant noise immunity and signal to noise ratio improvements. Although our discussion here is based on using a broadband incoherent source, CSK can also use coherent sources, either a short pulsed laser or an array of CW laser sources. To date most implementations of CSK, including CDMA, have used free-space bulk optics, and are not compatible with the compact packaging requirements, stability, and integration needed by telecommunications industry. In this invention, we present a design for an integrated, programmable, CSK encoder/decoder in a compact reflective symmetric structure as a new device for optical CDMA application.
The first bipolar coding technique applied to an incoherent spectrum-encoded optical CDMA system was demonstrated by James Young""s group at Rice University. (Young, et al, U.S. Pat. No. 5,760,941) (FIG. 3)
FIG. 3 shows the basic spectral encoder/decoder of the present invention. As an encoder, a super-fluorescent fiber source (SFS) of light 50 is incident on a spectral demultiplexer (an optical grating in this case) 51, which spreads the light into a band 52 of light of different wavelengths (colors). The light band 52 is reflected by a first mirror 55 to pass through a coded mask 53. The mask 53 allows certain wavelengths of the reflected light from the mirror 55 to be transmitted and other wavelengths of light to be reflected. The transmitted wavelengths are reflected by a second mirror 54 and focused on the spectral multiplexer (grating) 51 to generate a single light beam 56 as spectral code for digital 1""s. The reflected wavelengths from the mask 53 are reflected by the first mirror 55 and focused on the grating 51 to generate a single light beam 57 as spectral code for digital 0""s. These digital codes are modulated
FIG. 3 can also be used as a decoder. In this case, the incident light 50 for an encoder is replaced as an incoming signal, which is coded. The functions of the rest of the components are the same as that for an encoder. Only the incoming coded signal which correlates with the code of the mask 53 can yield 1""s output as ray 56 and 0""s output as ray 57.
The use of bulky optics introduces mismatch problems among encoder and decoder that affects the correlation detection process, such as the resolution and precision of the gratings, coding masks, etc.
In this invention, we utilize the planar lightwave circuit technology to implement an integrated device for this application. We will use the above system as an example to explain our concept, our invention is suitable for other types of systems stated above as well. The essential components for all required functions of a typical O-CDMA encoder/decoder can be described in FIGS . 1-2.
FIGS. 1-2 show the basic components and system of an O-CDMA system. FIG. 1 shows an O-CDMA encoding scheme. A broadband light source 10 is incident on an array waveguide grating (AWG) 11 to spread the light source into many different colors. Since the light source intensity throughout the spectrum may not be uniform, the different color lights from the grating 11 are equalized in intensity by the attenuators 12 before passing through the coded mask 13 (indicated as xe2x80x9cEncoding Controlxe2x80x9d). The coded mask 13 transmits certain wavelengths of light and reflects other wavelengths of light. The transmitted wavelengths may represent digital 1""s and the reflected wavelengths may represent digital 0""s. Each set of 1""s light is focused by grating 14 to generate a single light beam 16, which has a spectral code for digital xe2x80x9c1xe2x80x9d. The set of 0""s light is focused by a grating 15 to generate a single light beam 17 for spectral code xe2x80x9c0xe2x80x9d. The output of the digital communication signal is done through the section and transmission of either a xe2x80x9c1xe2x80x9d signal or a xe2x80x9c0xe2x80x9d signal for each clock cycle. This splitting into two groups is referred to as a 1xc3x972 switch. These two groups of spectral codes are alternately switched (modulated) with digital data 19 in a high-speed switch (modulator) 18. The output of the high speed switch 18 the coded signal 20 sent out to be transmitted.
At a receiving station, the coded signal can be decoded as shown in FIG. 2. The coded signal 30 irradiates a spectral demultiplexer 31 to spread the signal into different wavelengths. The different color signals pass through a coded mask 33, which correlates the incoming signal with the particular code of the mask 33. Certain wavelength components of the correlated signal are transmitted as digital 1""s in one path. Other wavelength components are transmitted as digital 0""s in another path. The 1""s signals are combined on a spectral multiplexer 34 to generate a light beam 36 for spectral code xe2x80x9c1xe2x80x9d. The 0""s signals are combined by another multiplexer 35 to generate a light beam 37 for spectral code xe2x80x9c0xe2x80x9d. The 1xe2x80x2 light beam 36 irradiates a photodiode P1. The 0""s light beam 37 irradiates a photodiode P2. The outputs of P1 and P2 are fed to a balanced detector 38, which yields a data output 40. Any uncorrelated signals appear as a noise and is canceled by the balanced detector 38.
The scheme presented in FIGS. 1-2 offers a bipolar signaling transmission for optical CDMA systems. However, using individual gratings, attenuators and switches will make devices too complex to be integrated. We take advantage of the symmetric structures and realize the encoder/decoder in the description of the invention.
An object of this invention is to integrate an O-CDMA system on a monolithic chip. Another object of this invention is to produce uniform spectral amplitude in the processing the optical signal of un-uniform spectrum. Still another object of this invention is to provide programmable capability of the O-CDMA system. A further object of this invention is to provide a combined optical multiplexer/demultiplexer for encoding and decoding for at least one optical signal.
These objects are achieved by using the same AWG both for the incoming signal and the outgoing signal. An incoming full spectrum signal is split into two paths by a 3-dB coupler. The signal in each path is decomposed into numerous spectral components and correlated with a coded mask. A phase-shifter is inserted in every one of the paths. The split signals are then reflected by a common mirror. The two reflected signals are then combined to form a Mach-Zehnder interferometer (MZI) switch. The interference of the phase-shifted signal in one path and the signal without phase-shift in the other path yield spectral code for digital xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d. After digitization by the MZI switch, the spectral components make another pass at the same AWG to be focused to yield single output beam.